I believe the usage in the expression would come from parts 2 and 3 above, where the vein is a pathway, the idea being that the thought being introduced by the expression "in the same vein" takes the same or a similar pathway as the referenced idea. I can't imagine how "vain" would be worked into the usage.

A useful resource for this sort of question is the Collins Cobuild Concordance, a searchable database of English sentences used as the basis of their dictionaries: search it for "in the same vein|vain" to match both forms, and it has nearly 20 uses of "vein" and none of "vain".

Since this zombie has risen from the grave, it's worth pointing out that Google gives 13M results for vein vs 270K results for vain. Either the OP didn't do a very good job of searching, or we are witnessing the march of progress before our very eyes.

buck naked or butt naked? I always thought the latter but a lot of people use the former.

I'm 63 and only heard or used the term "buck naked" since childhood. The internet is full of speculation as to where the term came from, including that it may have originally morphed from "butt naked", which doesn't seem likely. I never heard "butt naked" until recent years.

buck naked or butt naked? I always thought the latter but a lot of people use the former.

You are correct, at least in my neck of the woods. I've never heard "buck naked" before.

The original form (and still the correct one, at least here in the UK) is "buck naked". It seems to have mutated in recent years, in the same way that "off your own bat" is often mangled into "off your own back".

The original form (and still the correct one, at least here in the UK) is "buck naked". It seems to have mutated in recent years, in the same way that "off your own bat" is often mangled into "off your own back".

What does this phrase even mean? Did Brits frequently own carrier bats to deliver messages to one another? So taking one person's idea for your own would be met with the derisive comment, "You clearly didn't develop that proposal off you your own bat"--meaning "You've been intercepting other peoples' carrier bats and reading the messages!"

The original form (and still the correct one, at least here in the UK) is "buck naked". It seems to have mutated in recent years, in the same way that "off your own bat" is often mangled into "off your own back".

What does this phrase even mean? Did Brits frequently own carrier bats to deliver messages to one another? So taking one person's idea for your own would be met with the derisive comment, "You clearly didn't develop that proposal off you your own bat"--meaning "You've been intercepting other peoples' carrier bats and reading the messages!"

I have thought that this may have been a term used regarding enslaved African men. And thus, it has morphed into "butt naked" attempting to be sensitive to that possibility (political correctness . . . just in case). It appears that the male deer is what most people are thinking of (not perhaps first on the mind of city folk) -- so I'd say I prefer "buck naked" and the latter is just a new use or, as mentioned, an attempt to not offend.

While "buck" historically can mean a black man, it has dozens of other meanings. The Historical Dictionary of American Slang lists "buck-bathing" with the derivation "buck (naked) + bathing". So "buck" by itself can mean naked which seems the obvious derivation of "buck naked".

I have thought that this may have been a term used regarding enslaved African men. And thus, it has morphed into "butt naked" attempting to be sensitive to that possibility (political correctness . . . just in case). It appears that the male deer is what most people are thinking of (not perhaps first on the mind of city folk) -- so I'd say I prefer "buck naked" and the latter is just a new use or, as mentioned, an attempt to not offend.

My bold.

Yes, this is the origin of the phrase "buck naked," i.e., standing naked and vulnerable on the auction block (as in "black buck"). Sad, not politically correct, but true.